This invention relates generally to a way to improve the manufacture of individual battery cells into assembled modules, and more particularly to the assembly of prismatically-shaped cans of cells using high-speed battery stacking.
The increasing demand to improve vehicular fuel economy and reduce vehicular emissions has led to the development of both hybrid vehicles and pure electric vehicles. Pure electric vehicles may be powered by a battery pack (which is made up of numerous smaller modules or cells), while hybrid vehicles include two or more energy sources, such as a gasoline (also referred to as an internal combustion) engine used as either a backup to or in cooperation with a battery pack. There are two broad versions of hybrid vehicles currently in use. In a first version (known as a charge-depleting hybrid architecture), the battery can be charged off a conventional electrical grid such as a 120 VAC or 240 VAC power line. In a second version (known as a charge-sustaining hybrid architecture), the battery receives all of its electrical charging from one or both of the internal combustion engine and regenerative braking. In one form of either version, the pack is made from numerous modules, which in turn are made up of numerous individual cells.
Typically, the individual cells that make up a module are of a generally rectangular, planar (or prismatic) structure that includes alternating stacks of sheet-like positive and negative electrodes having a similarly-shaped electrolytic separator disposed between each positive and negative electrode pair; these separators are used to prevent physical contact between positive electrodes and negative electrodes within each cell while enabling ionic transport between them. In one form, the separators are configured to absorb the liquid electrolyte of the cell. Cooling features are also frequently employed to convey away the heat generated by the various individual cells during the charging and discharging activities associated with battery operation; in one form, such cooling features may be formed as yet another generally planar sheet-like device that can be added between the various cells as part of the stacked arrangement of components that make up the module. Connection tabs extend from a peripheral edge of each cell to allow mechanical and electrical connection between the electrodes of the individual battery cells. Proper alignment of the various tabs is generally required to ensure low electrical resistance to bus bars or related conductors, as well as for robust mechanical connectivity. These prismatic cells typically have either a soft, flexible case (called “pouch” cells) or a hard rigid case (called “can” or “cannular” cells). Depending on the application, the individual battery cells may be arranged in series, parallel or combinations thereof to produce the desired voltage and capacity. Numerous frames, trays, covers and related structure may be included to provide support for the various cells, modules and packs, and as such help to define a larger assembly of such cells, modules or packs.
Due to the prismatic dimensions, the current common practice for handling the rigid cannular cells during assembly is by stacking them along a generally vertical axis (for example, along the so-called y-axis in the well-known Cartesian coordinate system) such that the cells and frames are loaded with their largest flat surfaces laying down. However, the slightly bulged flat cells and the nesting geometries of the frames require them be stacked with the subassemblies standing up on their narrow, but flat edge surfaces. The cells may become bulged for various reasons; one such reason is due to increases in mechanical pressure that may arise from electrode expansion during operation that presses on the can walls, or internal gaseous pressure. In one particular instance, such expansion may be caused by electrolyte evaporation as heat is generated during operation, while in another, electrochemical reactions within the cell may create gaseous byproducts. As such, changes in stacking orientation may be required. Unfortunately, such changes in orientation can be a complex, expensive and inefficient process.
In one form, it is known to manufacture a battery module assembly by using robotic pick-and-place component transport systems. Such approaches remove the cells from the shipping dunnage, transfer the cells via conveyor to an initial process step (typically in the form of electrical verification) and then transfer them via robotic pick-and-place equipment to the high precision carrier. Such approaches are useful for assembling layered cells that have tight placement tolerance requirements, as well as those with special handling needs. While this method is effective for protecting the cell during the assembly operation, it also leads to expensive tooling and wasted assembly time to locate the carrier in position, remove the part for the specific station operation and then return the part to the carrier to move to the next operation. This in turn forces packaging and tooling operations to become more complex and expensive.
A previous horizontal battery stacking mechanism, which is described in co-pending application entitled LARGE FORMAT CELL HANDLING FOR HIGH SPEED ASSEMBLY, application Ser. No. 13/835,858 filed on Mar. 15, 2013 that is owned by the Assignees of the present invention and incorporated herein by reference, discloses the use of a conveyor belt with cams, lifters and guides to enable high speed assembly for large format cells that go through cell re-orientation and part sequencing steps. While useful for its intended purpose, the cams and the lifters that move in response to the cams still need to go through retracting and recirculating movements once the assemblies have been pressed together at the stacking stand. This in turn requires that the lifters, cam-followers and related equipment be returned to the place where they first engage the assemblies; during this return trip, they are not being used to help the assemblies being carried along the system.
What is needed is a battery stacking approach that permits low cost, high speed continuous assembly that eliminates the need for high precision packaging and tooling, and that allows for reduced part cost by permitting larger dimensional variation. A battery stacking system employing such an approach would also occupy a relatively small manufacturing floor space footprint.